In the conventional storage type, gas-fired water heater, a gas burner is located beneath the lower head of the tank and waste gases of combustion from the burner are discharged through one or more vertical flues that extend upwardly through the tank. With this construction, heat is transferred from the burner to the lower head to heat the water in the tank and heat is also transferred to the water from the waste gases passing upwardly through the flues.
In an attempt to increase the efficiency of the water heater, submersible chamber heaters have been developed. With submersible chamber heaters, the burner, instead of being mounted beneath the lower head, is positioned in a closed combustion chamber located within the lower portion of the tank. Waste gases from the combustion process are discharged through a flue which extends upwardly through the tank as disclosed in U.S. Pat. No. 4,301,772, and the gases flowing upwardly within the flue act to transfer additional heat to the water in the tank.
During standby periods when the burner is not in operation, there will be a flow of air by convection through the internal flue with the resultant transfer of heat from the heated water in the tank to the air passing upwardly through the flue, thereby resulting in considerable heat loss during standby periods. Because of this, the conventional water heater using internal flues has an in service efficiency of less than 70%.
There is also a tendency in the conventional water heater utilizing internal flues for the water to stratify and be overheated. Heated water will rise to the top of the tank by convection and during periods of short intermittent draws, the heated water in the upper end of the tank may be overheated by heat transfer from the hot flue gases passing through the internal flues. With commercial water heaters, where it is desired to heat water to sanitizing temperatures of 180.degree. F., it is difficult to heat the water to this temperature without overheating and resultant actuation of the temperature-pressure relief valve.